Bowed rotor start using direct temperature measurement

ABSTRACT

A bowed rotor start mitigation system for a gas turbine engine is provided. The bow rotor start mitigation system includes a controller operable to receive a speed input indicative of a rotor speed of the gas turbine engine and a measured temperature of the gas turbine engine. The controller is further operable to drive motoring of the gas turbine engine by oscillating the rotor speed within a motoring band for a motoring time based on the measured temperature when a start sequence of the gas turbine engine is initiated.

BACKGROUND

This disclosure relates to gas turbine engines, and more particularly toan apparatus, system and method for mitigating a bowed rotor startcondition using direct temperature measurement in a gas turbine engine.

Gas turbine engines are used in numerous applications, one of which isfor providing thrust to an airplane. When the gas turbine engine of anairplane has been shut off for example, after an airplane has landed atan airport, the engine is hot and due to heat rise, the upper portionsof the engine will be hotter than lower portions of the engine. Whenthis occurs thermal expansion may cause deflection of components of theengine which may result in a “bowed rotor” condition. If a gas turbineengine is in such a “bowed rotor” condition it is undesirable to restartor start the engine.

Accordingly, it is desirable to provide a method and/or apparatus fordetecting and preventing a “bowed rotor” condition.

BRIEF DESCRIPTION

In an embodiment, a bowed rotor start mitigation system for a gasturbine engine is provided. The bow rotor start mitigation systemincludes a controller operable to receive a speed input indicative of arotor speed of the gas turbine engine and a measured temperature of thegas turbine engine. The controller is further operable to drive motoringof the gas turbine engine by oscillating the rotor speed within amotoring band for a motoring time based on the measured temperature whena start sequence of the gas turbine engine is initiated.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the motoring band includes a range of speeds below aresonance speed of the gas turbine engine.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the measured temperature is adjusted with respect to ameasured ambient temperature of the gas turbine engine.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the motoring time is associated with a bowed rotor riskparameter that is determined based on the measured temperature.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the measured temperature is determined based on readingone or more temperature sensors of the gas turbine engine for apredetermined period of time when the start sequence of the gas turbineengine is initiated.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where based on determining that bowed rotor mitigation iscomplete, the controller is operable to monitor the vibration levelwhile sweeping through a range of rotor speeds including a criticalrotor speed and determine whether bowed rotor mitigation was successfulprior to starting the gas turbine engine.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the measured temperature is determined based on readingdata from one or more temperature sensors at station 3 of the gasturbine engine.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the measured temperature is determined based on readingdata from one or more temperature sensors at station 4 of the gasturbine engine.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the measured temperature is determined based on readingdata from one or more temperature sensors at station 4.5 of the gasturbine engine.

According to an embodiment, a gas turbine engine includes a motoringsystem operable to drive rotation of the gas turbine engine, a speedpickup, a temperature sensor, and an electronic engine control operableto receive a speed input from the speed pickup indicative of a rotorspeed of the gas turbine engine and a measured temperature from thetemperature sensor. The electronic engine control is further operable todrive motoring of the gas turbine engine by controlling the motoringsystem to oscillate the rotor speed within a motoring band for amotoring time based on the measured temperature when a start sequence ofthe gas turbine engine is initiated.

According to an embodiment, a method of bowed rotor start mitigation fora gas turbine engine includes receiving, by a controller, a speed inputindicative of a rotor speed of the gas turbine engine. The controlleralso receives a measured temperature of the gas turbine engine. Thecontroller drives motoring of the gas turbine engine by oscillating therotor speed within a motoring band for a motoring time based on themeasured temperature when a start sequence of the gas turbine engine isinitiated.

A technical effect of the apparatus, systems and methods is achieved byusing a start sequence for a gas turbine engine as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the present disclosure isparticularly pointed out and distinctly claimed in the claims at theconclusion of the specification. The foregoing and other features, andadvantages of the present disclosure are apparent from the followingdetailed description taken in conjunction with the accompanying drawingsin which:

FIG. 1 is a cross-sectional view of a gas turbine engine;

FIG. 2 is a schematic illustration of a starting system for a gasturbine engine in accordance with an embodiment of the disclosure;

FIG. 3 is a schematic illustration of a starting system for a gasturbine engine in accordance with another embodiment of the disclosure;

FIG. 4 is a block diagram of a system for bowed rotor start mitigationin accordance with an embodiment of the disclosure;

FIG. 5 is a flow chart illustrating a method of bowed rotor startmitigation using direct temperature measurement of a gas turbine enginein accordance with an embodiment of the disclosure;

FIG. 6 is a graph illustrating a bowed rotor risk score with respect totime in accordance with an embodiment of the disclosure;

FIG. 7 is a graph illustrating starting profiles of a normal start and abowed rotor mitigation sequence start in accordance with an embodimentof the disclosure;

FIG. 8 is a graph illustrating examples of various vibration levelprofiles of an engine in accordance with an embodiment of thedisclosure;

FIG. 9 is a schematic illustration of a high spool gas path with astraddle-mounted spool in accordance with an embodiment of thedisclosure;

FIG. 10 is a schematic illustration of a high spool gas path with anoverhung spool in accordance with an embodiment of the disclosure;

FIG. 11 is a graph illustrating commanded starter valve opening withrespect to time in accordance with an embodiment of the disclosure; and

FIG. 12 is a graph illustrating a target rotor speed profile of a drymotoring profile and an actual rotor speed versus time in accordancewith an embodiment of the disclosure.

DETAILED DESCRIPTION

Various embodiments of the present disclosure are related to a bowedrotor start mitigation system in a gas turbine engine. Embodiments caninclude using a measured temperature value of the gas turbine engine toestimate heat stored in the engine core when a start sequence isinitiated and identify a risk of a bowed rotor. The measured temperaturevalue alone or in combination with other values can be used to calculatea bowed rotor risk parameter. For example, the measured temperature canbe adjusted relative to an ambient temperature when calculating thebowed rotor risk parameter. The bowed rotor risk parameter may be usedto take a control action to mitigate the risk of starting the gasturbine engine with a bowed rotor. The control action can include drymotoring, which may be performed by running an engine starting system ata lower speed with a longer duration than typically used for enginestarting. Specifically, the dry motoring speed referred to herein is aspeed below a major resonance speed where, if the temperatures areunhomogenized, the combination of the bowed rotor and similarly bowedcasing and the resonance speed would lead to high amplitude oscillationin the rotor and high rubbing of blade tips on one side of the rotor,especially in the high pressure compressor if the rotor isstraddle-mounted. The engine speed during dry motoring may oscillatewithin a motoring band of speed by modulating a starter valve, forexample. Some embodiments increase the rotor speed of the starting spoolto approach a critical rotor speed gradually according to a dry motoringprofile and as thermal distortion is decreased they then acceleratebeyond the critical rotor speed to complete the engine starting process.

A full authority digital engine control (FADEC) system or other systemmay send a message to the cockpit to inform the crew of an extended timestart time due to bowed rotor mitigation actions prior to completing anengine start sequence. If the engine is in a ground test or in a teststand, a message can be sent to the test stand or cockpit based on thecontrol-calculated risk of a bowed rotor. A test stand crew can bealerted regarding a requirement to keep the starting spool of the engineto a speed below the known resonance speed of the rotor in order tohomogenize the temperature of the rotor and the casings about the rotorwhich also are distorted by temperature non-uniformity.

Monitoring of vibration signatures during the engine starting sequencecan also or separately be used to assess the risk that a bowed rotorstart has occurred due to some system malfunction and then directmaintenance, for instance, in the case of suspected outer air seal rubespecially in the high compressor. Vibration data for the engine canalso be monitored after bowed rotor mitigation is performed during anengine start sequence to confirm success of bowed rotor mitigation. Ifbowed rotor mitigation is unsuccessful or determined to be incomplete bythe FADEC, resulting metrics (e.g., time, date, global positioningsatellite (GPS) coordinates, vibration level vs. time, etc.) of theattempted bowed rotor mitigation can be recorded and/or transmitted todirect maintenance

During a dry motoring sequence, a starter air pressure valve can bemodulated or otherwise dynamically adjusted to limit high rotor speedbelow high spool resonance speed and prevent rub during dry motoringoperation. The bowed rotor risk parameter can also be used to limit drymotoring duration to reduce the impact on air starter turbine life. Themonitoring of vibration signatures during the entire engine startingsequence can also or separately be used to assess the risk of a bowedrotor start and direct maintenance, for instance, in the case ofsuspected outer air seal rub especially in the high compressor.

Referring now to FIG. 1, a schematic illustration of a gas turbineengine 10 is provided. The gas turbine engine 10 has among othercomponents a fan through which ambient air is propelled into the enginehousing, a compressor for pressurizing the air received from the fan anda combustor wherein the compressed air is mixed with fuel and ignitedfor generating combustion gases. The gas turbine engine 10 furthercomprises a turbine section for extracting energy from the combustiongases. Fuel is injected into the combustor of the gas turbine engine 10for mixing with the compressed air from the compressor and ignition ofthe resultant mixture. The fan, compressor, combustor, and turbine aretypically all concentric about a central longitudinal axis of the gasturbine engine 10. Thus, thermal deflection of the components of the gasturbine engine 10 may create the aforementioned bowing or “bowed rotor”condition along the common central longitudinal axis of the gas turbineengine 10 and thus it is desirable to clear or remove the bowedcondition prior to the starting or restarting of the gas turbine engine10.

FIG. 1 schematically illustrates a gas turbine engine 10 that can beused to power an aircraft, for example. The gas turbine engine 10 isdisclosed herein as a multi-spool turbofan that generally incorporates afan section 22, a compressor section 24, a combustor section 26 and aturbine section 28. The fan section 22 drives air along a bypassflowpath while the compressor section 24 drives air along a coreflowpath for compression and communication into the combustor section 26then expansion through the turbine section 28. Although depicted as aturbofan gas turbine engine in the disclosed non-limiting embodimentwith two turbines and is sometimes referred to as a two spool engine, itshould be understood that the concepts described herein are not limitedto use with turbofans as the teachings may be applied to other types ofturbine engines including three-spool architectures. In both of thesearchitectures the starting spool is that spool that is located aroundthe combustor, meaning the compressor part of the starting spool isflowing directly into the combustor and the combustor flows directlyinto the turbine section.

The engine 10 generally includes a low speed spool 30 and a high speedspool 32 mounted for rotation about an engine central longitudinal axisA relative to an engine static structure 36 via several bearing systems38. It should be understood that various bearing systems 38 at variouslocations may alternatively or additionally be provided.

The low speed spool 30 generally includes an inner shaft 40 thatinterconnects a fan 42, a low pressure compressor 44 and a low pressureturbine 46. The inner shaft 40 is connected to the fan 42 through ageared architecture 48 to drive the fan 42 at a lower speed than the lowspeed spool 30 in the example of FIG. 1. The high speed spool 32includes an outer shaft 50 that interconnects a high pressure compressor52 and high pressure turbine 54. A combustor 56 is arranged between thehigh pressure compressor 52 and the high pressure turbine 54. Amid-turbine frame 57 of the engine static structure 36 is arrangedgenerally between the high pressure turbine 54 and the low pressureturbine 46. The mid-turbine frame 57 further supports bearing systems 38in the turbine section 28. The inner shaft 40 and the outer shaft 50 areconcentric and rotate via bearing systems 38 about the engine centrallongitudinal axis A which is collinear with their longitudinal axes.

The core airflow is compressed by the low pressure compressor 44 thenthe high pressure compressor 52, mixed and burned with fuel in thecombustor 56, then expanded over the high pressure turbine 54 and lowpressure turbine 46. The mid-turbine frame 57 includes airfoils 59 whichare in the core airflow path. The turbines 46, 54 rotationally drive therespective low speed spool 30 and high speed spool 32 in response to theexpansion.

A number of stations for temperature and pressuremeasurement/computation are defined with respect to the gas turbineengine 10 according to conventional nomenclature. Station 2 is at aninlet of low pressure compressor 44 having a temperature T2 and apressure P2. Station 2.5 is at an exit of the low pressure compressor 44having a temperature T2.5 and a pressure P2.5. Station 3 is at an inletof the combustor 56 having a temperature T3 and a pressure P3. Station 4is at an exit of the combustor 56 having a temperature T4 and a pressureP4. Station 4.5 is at an exit of the high pressure turbine 54 having atemperature T4.5 and a pressure P4.5. Station 5 is at an exit of the lowpressure turbine 46 having a temperature T5 and a pressure P5. Measuredtemperatures in embodiments may be acquired at one or more stations 2-5.For example, temperature T3 at station 3 can be used to as an enginerotor temperature measurement when there is no engine rotation.Alternatively, if available, temperature values at stations 4 (T4), 4.5(T4.5), and/or 5 (T5) can be used as an engine rotor temperature.Temperature measurements can be normalized to account for hot day/coldday differences. For instance, temperature T2 can be used as an ambienttemperature and a measured temperature (e.g., T3) can be normalized bysubtracting temperature T2.

Although FIG. 1 depicts one example configuration, it will be understoodthat embodiments as described herein can cover a wide range ofconfigurations. For example, embodiments may be implemented in aconfiguration that is described as a “straddle-mounted” spool 32A ofFIG. 9. This configuration places two bearing compartments 37A and 39A(which may include a ball bearing and a roller bearing respectively),outside of the plane of most of the compressor disks of high pressurecompressor 52A and at outside at least one of the turbine disks of highpressure turbine 54A. In contrast with a straddle-mounted spoolarrangement, other embodiments may be implemented using an over-hungmounted spool 32B as depicted in FIG. 10. In over-hung mounted spool32B, a bearing compartment 37B is located forward of the first turbinedisk of high pressure turbine 54B such that the high pressure turbine54B is overhung, and it is physically located aft of its main supportingstructure. The use of straddle-mounted spools has advantages anddisadvantages in the design of a gas turbine, but one characteristic ofthe straddle-mounted design is that the span between the bearingcompartments 37A and 39A is long, making the amplitude of the high spotof a bowed rotor greater and the resonance speed that cannot betransited prior to temperature homogenization is lower. For any thrustrating, the straddle mounted arrangement, such as straddle-mounted spool32A, gives Lsupport/Dhpt values that are higher, and the overhungmounted arrangement, such as overhung spool 32B, can be as much as 60%of the straddle-mounted Lsupport/Dhpt. Lsupport is the distance betweenbearings (e.g., between bearing compartments 37A and 39A or betweenbearing compartments 37B and 39B), and Dhpt is the diameter of the lastblade of the high pressure turbine (e.g., high pressure turbine 54A orhigh pressure turbine 54B). As one example, a straddle-mounted enginestarting spool, such as straddle-mounted spool 32A, with a rollerbearing at bearing compartment 39A located aft of the high pressureturbine 54A may be more vulnerable to bowed rotor problems since theLsupport/Dhpt ranges from 1.9 to 5.6. FIGS. 9 and 10 also illustrate astarter 120 interfacing via a tower shaft 55 with the straddle-mountedspool 32A proximate high compressor 52A and interfacing via tower shaft55 with the overhung mounted spool 32B proximate high compressor 52B aspart of a starting system.

Turning now to FIG. 2, a schematic of a starting system 100 for the gasturbine engine 10 of FIG. 1 is depicted according to an embodiment. Thestarting system 100 is also referred to generally as a gas turbineengine system. In the example of FIG. 2, the starting system 100includes a controller 102 which may be an electronic engine control,such as a dual-channel FADEC, and/or engine health monitoring unit. Inan embodiment, the controller 102 may include memory to storeinstructions that are executed by one or more processors. The executableinstructions may be stored or organized in any manner and at any levelof abstraction, such as in connection with a controlling and/ormonitoring operation of the engine 10 of FIG. 1. The one or moreprocessors can be any type of central processing unit (CPU), including ageneral purpose processor, a digital signal processor (DSP), amicrocontroller, an application specific integrated circuit (ASIC), afield programmable gate array (FPGA), or the like. Also, in embodiments,the memory may include random access memory (RAM), read only memory(ROM), or other electronic, optical, magnetic, or any other computerreadable medium onto which is stored data and control algorithms in anon-transitory form.

The starting system 100 can also include a data storage unit (DSU) 104that retains data between shutdowns of the gas turbine engine 10 ofFIG. 1. The DSU 104 includes non-volatile memory and retains databetween cycling of power to the controller 102 and DSU 104. Acommunication link 106 can include an aircraft and/or test standcommunication bus to interface with aircraft controls, e.g., a cockpit,various onboard computer systems, and/or a test stand.

A motoring system 108 is operable to drive rotation of a starting spool(e.g., high speed spool 32) of the gas turbine engine 10 of FIG. 1.Either or both channels of controller 102 can alternate on and offcommands to an electromechanical device 110 coupled to a discretestarter valve 116A to achieve a partially open position of the discretestarter valve 116A to control a flow from a starter air supply 114 (alsoreferred to as air supply 114) through a transfer duct 118 to an airturbine starter 120 (also referred to as starter 120 or pneumaticstarter motor 120) to drive rotation of a starting spool of the gasturbine engine 10 below an engine idle speed. The air supply 114 (alsoreferred to as starter air supply 114) can be provided by any knownsource of compressed air, such as an auxiliary power unit or groundcart.

The controller 102 can monitor a speed sensor, such as speed pickup 122that may sense the speed of the engine rotor through its connection to agearbox 124 which is in turn connected to the high speed spool 32 viatower shaft 55 (e.g., rotational speed of high speed spool 32) or anyother such sensor for detecting or determining the speed of the gasturbine engine 10 of FIG. 1. The starter 120 may be coupled to thegearbox 124 of the gas turbine engine 10 of FIG. 1 directly or through atransmission such as a clutch system (not depicted). The controller 102can establish a control loop with respect to rotor speed to adjustpositioning of the discrete starter valve 116A.

The discrete starter valve 116A is an embodiment of a starter valve thatis designed as an on/off valve which is typically commanded to eitherfully opened or fully closed. However, there is a time lag to achievethe fully open position and the fully closed position. By selectivelyalternating an on-command time with an off-command time through theelectromechanical device 110, intermediate positioning states (i.e.,partially opened/closed) can be achieved. The controller 102 canmodulate the on and off commands (e.g., as a duty cycle using pulsewidth modulation) to the electromechanical device 110 to further openthe discrete starter valve 116A and increase a rotational speed of thestarting spool of the gas turbine engine 10 of FIG. 1. In an embodiment,the electromechanical device 110 has a cycle time defined between anoff-command to an on-command to the off-command that is at least half ofa movement time for the discrete starter valve 116A to transition fromfully closed to fully open. Pneumatic lines 112A and 112B or amechanical linkage (not depicted) can be used to drive the discretestarter valve 116A between the open position and the closed position.The electromechanical device 110 can be a solenoid that positions thediscrete starter valve 116A based on intermittently supplied electricpower as commanded by the controller 102. In an alternate embodiment,the electromechanical device 110 is an electric valve controlling muscleair to adjust the position of the discrete starter valve 116A ascommanded by the controller 102.

In the example of FIG. 2, the starting system 100 also includes avibration monitoring system 126. The vibration monitoring system 126includes at least one vibration pickup 128, e.g., an accelerometer,operable to monitor vibration of the gas turbine engine 10 of FIG. 1.Vibration signal processing 130 can be performed locally with respect tothe vibration pickup 128, within the controller 102, or through aseparate vibration processing system, which may be part of an enginehealth monitoring system to acquire vibration data 132. Alternatively,the vibration monitoring system 126 can be omitted in some embodiments.

One or more temperature sensors 134, such as thermocouples, can providemeasured temperatures at associated locations of the gas turbine engine10 to the controller 102. For example, the temperature sensors 134 canbe located at station 2 (T2), station 2.5 (T2.5), station 3 (T3),station 4 (T4), station 4.5 (T4.5), and/or station 5 (T5) as previouslydescribed with respect to FIG. 1.

Similar to FIG. 2, FIG. 3 is a schematic illustration of a startingsystem 100A for the gas turbine engine 10 of FIG. 1 in accordance withanother embodiment. The starting system 100A includes controller 102that controls motoring system 108A, as an alternate embodiment of themotoring system 108 of FIG. 2. Rather than using an electromechanicaldevice 110 coupled to a discrete starter valve 116A to achieve apartially open position of the discrete starter valve 116A of FIG. 2,the motoring system 108A of FIG. 3 uses a variable position startervalve 116B. In FIG. 3, either or both channels of controller 102 canoutput a valve control signal 150 operable to dynamically adjust a valveangle of the variable position starter valve 116A that selectivelyallows a portion of the air supply 114 to pass through the variableposition starter valve 116B and transfer duct 118 to air turbine starter120. The variable position starter valve 116B is a continuous/infinitelyadjustable valve that can hold a commanded valve angle, which may beexpressed in terms of a percentage open/closed and/or an angular value(e.g., degrees or radians). Performance parameters of the variableposition starter valve 116B can be selected to meet dynamic responserequirements of the starting system 100A. For example, in someembodiments, the variable position starter valve 116B has a responserate of 0% to 100% open in less than 40 seconds. In other embodiments,the variable position starter valve 116B has a response rate of 0% to100% open in less than 30 seconds. In further embodiments, the variableposition starter valve 116B has a response rate of 0% to 100% open inless than 20 seconds.

The controller 102 can monitor a valve angle of the variable positionstarter valve 116B using valve angle feedback signals 152 provided toboth channels of controller 102. As one example, in an active/standbyconfiguration, both channels of the controller 102 can use the valveangle feedback signals 152 to track a current valve angle, while onlyone channel designated as an active channel outputs valve control signal150. Upon a failure of the active channel, the standby channel ofcontroller 102 can take over as the active channel to output valvecontrol signal 150. In an alternate embodiment, both channels ofcontroller 102 output all or a portion of a valve angle commandsimultaneously on the valve control signals 150. The controller 102 canestablish an outer control loop with respect to rotor speed and an innercontrol loop with respect to the valve angle of the variable positionstarter valve 116B. One or more temperature sensors 134, such asthermocouples, can provide measured temperatures at associated locationsof the gas turbine engine 10 to the controller 102.

As in the example of FIG. 2, the starting system 100A of FIG. 3 alsoincludes vibration monitoring system 126. The vibration monitoringsystem 126 includes at least one vibration pickup 128, e.g., anaccelerometer, operable to monitor vibration of the gas turbine engine10 of FIG. 1. Vibration signal processing 130 can be performed locallywith respect to the vibration pickup 128, within the controller 102, orthrough a separate vibration processing system, which may be part of anengine health monitoring system to acquire vibration data 132.Alternatively, the vibration monitoring system 126 can be omitted insome embodiments.

FIG. 4 is a block diagram of a system 200 for bowed rotor startmitigation using direct temperature measurement that may control thediscrete starter valve 116A of FIG. 2 or the variable position startervalve 116B of FIG. 3 via control signals 210 in accordance with anembodiment. The system 200 may also be referred to as a bowed rotorstart mitigation system. In the example of FIG. 4, the system 200includes sensor processing 202 operable to acquire and condition datafrom a variety of sensors such as the speed pickup 122, vibration pickup128, and temperature sensors 134 of FIG. 2. Where the variable positionstarter valve 116B of FIG. 3 is used, the sensor processing 202 may alsoprovide valve angle 207 to motoring controller 208 based on the valveangle feedback 152 of FIG. 3. In the example of FIG. 4, sensorprocessing 202 provides compressor exit temperature T3 and ambienttemperature T2 to core temperature processing 204. Alternatively oradditionally, one or more temperature values from stations 4 (T4), 4.5(T4.5), and/or 5 (T5) can be provided to core temperature processing204. Sensor processing 202 can provide rotor speed N2 (i.e., speed ofhigh speed spool 32) to a motoring controller 208 and a mitigationmonitor 214. Sensor processing 202 may also provide vibration data 132to mitigation monitor 214. The sensor processing 202, core temperatureprocessing 204, motoring controller 208, and/or mitigation monitor 214may be part of controller 102.

The heat state of the engine 10 or T_(core) is determined by the coretemperature processing 204. When the gas turbine engine 10 has stoppedrotating (e.g., rotor speed N2 is zero), the compressor exit temperatureT3 may be substantially equal to T_(core). In some embodiments, T_(core)is set equal to T3-T2 to adjust the temperature with respect to themeasured ambient temperature of the gas turbine engine 10. Further,temperature readings from other stations of the gas turbine engine 10can be used to determine T_(core). Communication link 106 can providethe core temperature processing 204 with an indication 205 that a startsequence of the gas turbine engine 10 has been initiated. Once rotationof the gas turbine engine 10 begins, temperature readings can becollected for a predetermined period of time, such as ten seconds. Thetemperature readings, e.g., T3 or T3-T2, can be averaged as coretemperature T_(core) before the temperature starts to change due to airflow from engine rotation. The core temperature processing 204 candetermine a bowed rotor risk parameter that is based on the measuredtemperature using a mapping function or lookup table. The bowed rotorrisk parameter can have an associated motoring time defining ananticipated amount of time for the motoring controller 208 to sendcontrol signals 210 to electromechanical device 110 for controllingdiscrete starter valve 116A of FIG. 2 or a dry motoring profile tocontrol valve control signals 150 for controlling variable positionstarter valve 116B of FIG. 3. For example, a higher risk of a bowedrotor may result in a longer duration of dry motoring or an extended drymotoring profile to reduce a temperature gradient prior to starting thegas turbine engine 10 of FIG. 1.

The bowed rotor risk parameter may be quantified according to a profilecurve 402 selected from a family of curves 404 that align with observedaircraft/engine conditions that impact turbine bore temperature (e.g.,starting spool temperature) and the resulting bowed rotor risk asdepicted in the example graph 400 of FIG. 6.

As used herein, motoring of the engine 10 in a modified start sequencerefers to the turning of a starting spool by the starter 120 at areduced speed without introduction of fuel and an ignition source inorder to cool the engine 10 to a point wherein a normal start sequencecan be implemented without starting the engine 10 in a bowed rotorstate. In other words, cool or ambient air is drawn into the engine 10while motoring the engine 10 at a reduced speed in order to clear the“bowed rotor” condition, which is referred to as a dry motoring mode.

The motoring controller 208 uses a dynamic control calculation in orderto determine a required valve position of the starter valve 116A, 116Bused to supply an air supply or starter air supply 114 to the engine 10in order to limit the motoring speed of the engine 10 to a target speedN_(target) within a motoring band or following a dry motoring profiledue to the position of the starter valve 116A, 116B. The required valveposition of the starter valve 116A, 116B can be determined based upon anair supply pressure as well as other factors including but not limitedto ambient air temperature, parasitic drag on the engine 10 from avariety of engine driven components such as electric generators andhydraulic pumps, and other variables such that the motoring controller208 closes the loop for an engine motoring speed target N_(target) forthe required amount of time based on the output of the bowed rotor startrisk model 206. In one embodiment, the dynamic control of the valveposition (e.g., open state of the valve (e.g., fully open, ½ open, ¼open, etc.) in order to limit the motoring speed of the engine 10) iscontrolled via duty cycle control (on/off timing using pulse widthmodulation) of electromechanical device 110 for discrete starter valve116A.

Referring now to FIG. 7, a graph 450 illustrating starting profiles of anormal start 452 and a bowed rotor mitigation sequence start 454 isdepicted according to an embodiment. During the normal start 452, uponreceiving the indication 205 of FIG. 4 that a start sequence of the gasturbine engine 10 has been initiated, the controller 102 of FIG. 2 opensdiscrete starter valve 116A of FIG. 2 (or variable position startervalve 116B of FIG. 3) to enable the air supply 114 to drive rotation ofstarter 120 to get rotor speed N2 of the gas turbine engine 10 up toidle speed 456. In an embodiment, upon receiving the indication 205 ofFIG. 4, controller 102 monitors one or more temperature sensors 134 fora predetermined period of time 458 while rotor speed N2 is at or aboutzero RPM. If core temperature processing 204 of FIG. 4 determines thatbowed rotor mitigation is needed based on measured temperature, thenbowed rotor mitigation sequence start 454 is performed instead of normalstart 452.

In the example of FIG. 7, bowed rotor mitigation sequence start 454oscillates the rotor speed N2 within a motoring band 460 for a motoringtime. The motoring band 460 can be defined between a starter valve closelimit 462 (i.e., an upper N2 speed limit) and a starter valve re-openlimit 464 (i.e., a lower N2 limit). In the example of FIG. 7, thestarter valve close limit 462 is about 3000 RPM and the starter valvere-open limit 464 is about 2000 RPM. The actual values used for thestarter valve close limit 462 and the starter valve re-open limit 464can be tuned to the specific performance characteristics of the gasturbine engine 10 as further described with respect to FIG. 8.

In some embodiments, an anticipated amount of dry motoring time can beused to determine a target rotor speed profile in a dry motoring profilefor the currently observed conditions. As one example, one or morebaseline characteristic curves for the target rotor speed profile can bedefined in tables or according to functions that may be rescaled toalign with the observed conditions. An example of a target rotor speedprofile 1002 is depicted in graph 1000 of FIG. 12 that includes a steepinitial transition portion 1004, followed by a gradually increasingportion 1006, and a late acceleration portion 1008 that increases rotorspeed above a critical rotor speed, through a fuel-on speed and anengine idle speed. The target rotor speed profile 1002 can be rescaledwith respect to time and/or select portions (e.g., portions 1004, 1006,1008) of the target rotor speed profile 1002 can be individually orcollectively rescaled (e.g., slope changes) with respect to time toextend or reduce the total motoring time. The target rotor speed profile1002 may include all positive slope values such that the actual rotorspeed 1010 is driven to essentially increase continuously while bowedrotor start mitigation is active. While the example of FIG. 12 depictsone example of the target rotor speed profile 1002 that can be definedin a dry motoring profile, it will be understood that many variationsare possible in embodiments.

The example of FIG. 11 illustrates how a valve angle command 902 can beadjusted between 0 to 100% of a commanded starter valve opening togenerate the actual rotor speed 1010 of FIG. 12. As the actual rotorspeed 1010 tracks to the steep initial transition portion 1004 of thetarget rotor speed profile 1002, the valve angle command 902 transitionsthrough points “a” and “b” to fully open the variable position startervalve 116B. As the slope of the target rotor speed profile 1002 isreduced in the gradually increasing portion 1006, the valve anglecommand 902 is reduced between points “b” and “c” to prevent the actualrotor speed 1010 from overshooting the target rotor speed profile 1002.In some embodiments, decisions to increase or decrease the commandedstarter valve opening is based on monitoring a rate of change of theactual rotor speed 1010 and projecting whether the actual rotor speed1010 will align with the target rotor speed profile 1002 at a futuretime. If it is determined that the actual rotor speed 1010 will notalign with the target rotor speed profile 1002 at a future time, thenthe valve angle of the variable position starter valve 116B is adjusted(e.g., increase or decrease the valve angle command 902) at acorresponding time. In the example of FIGS. 11 and 12, the valve anglecommand 902 oscillates with a gradually reduced amplitude between points“c”, “d”, and “e” as the actual rotor speed 1010 tracks to the targetrotor speed profile 1002 through the gradually increasing portion 1006.As dry motoring continues, the overall homogenization of the engine 10increases, which allows the actual rotor speed 1010 to safely approachthe critical rotor speed without risking damage. The valve angle commandtransitions from point “e” to point “f” and beyond to further increasethe actual rotor speed 1010 in the late acceleration portion 1008 abovethe critical rotor speed, through a fuel-on speed and an engine idlespeed. By continuously increasing the actual rotor speed 1010 during drymotoring, the bowed rotor condition can be reduced faster than holding aconstant slower speed.

In reference to FIGS. 4 and 12, the mitigation monitor 214 of FIG. 4 canoperate in response to receiving a complete indicator 212 to run averification of the bowed rotor mitigation. The mitigation monitor 214can provide mitigation results 216 to the motoring controller 208 andmay provide result metrics 218 to other systems, such a maintenancerequest or indicator. Peak vibrations can be checked by the mitigationmonitor 214 during the start processes to confirm that bowed rotormitigation successfully removed the bowed rotor condition. Themitigation monitor 214 may also run while dry motoring is active todetermine whether adjustments to the dry motoring profile are needed.For example, if a greater amount of vibration is detected than wasexpected, the mitigation monitor 214 can request that the motoringcontroller 208 reduce a slope of the target rotor speed profile 1002 ofFIG. 12 to extend the dry motoring time before driving the actual rotorspeed 1010 of FIG. 12 up to the critical rotor speed. Similarly, if themagnitude of vibration observed by the mitigation monitor 214 is lessthan expected, the mitigation monitor 214 can request that the motoringcontroller 208 increase a slope of the target rotor speed profile 1002of FIG. 12 to reduce the dry motoring time before driving the actualrotor speed 1010 of FIG. 12 up to the critical rotor speed.

FIG. 5 is a flow chart illustrating a method 300 of bowed rotor startmitigation using direct temperature measurement of a gas turbine enginein accordance with an embodiment. The method 300 of FIG. 5 is describedin reference to FIGS. 1-12 and may be performed with an alternate orderand include additional steps. At block 302, the controller 102 receivesreceive a speed input indicative of a rotor speed (e.g., N2) of the gasturbine engine 10. The speed input may be directly or indirectlyindicative of the rotational speed of high speed spool 32, for instance,derived from a rotational speed of gearbox 124, from speed pickup 122,or another source (not depicted).

At block 304, the controller 102 receives a measured temperature from atemperature sensor 134. The measured temperature can be determined basedon reading one or more temperature sensors 134 of the gas turbine engine10 for a predetermined period of time when a start sequence of the gasturbine engine is initiated (e.g., based on indication 205). Themeasured temperature may be adjusted with respect to a measured ambienttemperature of the gas turbine engine 10 (e.g., T3-T2, T4-T2, etc.).

At block 306, bowed rotor start mitigation can be performed usingmotoring system 108, 108A to rotate a starting spool of the gas turbineengine 10. The controller 102 can drive motoring of the gas turbineengine 10 by controlling the motoring system 108, 108A to oscillate therotor speed (e.g., N2) within a motoring band for a motoring time basedon the measured temperature when a start sequence of the gas turbineengine 10 is initiated. The motoring band can include a range of speedsbelow a resonance speed of the gas turbine engine 10. The motoring timecan be associated with a bowed rotor risk parameter that is determinedbased on the measured temperature. Alternatively, other motoringprofiles can be used, such as the target rotor speed profile 1002 forcontrolling starting spool speed during dry motoring.

At block 308, based on determining that bowed rotor mitigation iscomplete, the controller 102 can monitor the vibration level of the gasturbine engine 10 while sweeping through a range of rotor speedsincluding a critical rotor speed and determine whether bowed rotormitigation was successful prior to starting the gas turbine engine 10.The mitigation monitor 214 may receive a complete indicator 212 from themotoring controller 208 when the motoring controller 208 has completeddry motoring, for instance, if the motoring time has elapsed. If themitigation monitor 214 determines that the bowed rotor condition stillexists based on vibration data 132 collected, the motoring controller208 may restart dry motoring, or a maintenance request or indicator canbe triggered along with providing result metrics 218 for furtheranalysis. Metrics of attempted bowed rotor mitigation can be recorded inthe DSU 104 based on determining that the attempted bowed rotormitigation was unsuccessful or incomplete.

FIG. 8 is a graph illustrating examples of various vibration levelprofiles 502 of an engine, such as gas turbine engine 10 of FIG. 1. Thevibration level profiles 502 represent a variety of possible vibrationlevels observed before and/or after performing bowed rotor mitigation. Acritical speed 510 is the speed at which a vibration peak is expecteddue to amplification effects of a bowed rotor condition along with othercontributions to vibration level generally. A peak vibration 504 atcritical rotor speed 510 may be used to trigger different events. Forexample, if the peak vibration 504 at critical rotor speed 510 is belowa maintenance action threshold 506, then no further actions may beneeded. If the peak vibration 504 at critical rotor speed 510 is above adamage risk threshold 508, then an urgent maintenance action may berequested such as an engine check. If the peak vibration 504 at criticalrotor speed 510 is between the maintenance action threshold 506 and thedamage risk threshold 508, then further bowed rotor mitigation actionsmay be requested, such as extending/restarting dry motoring. In oneembodiment, a maintenance request is triggered based on the actualvibration level exceeding maintenance action threshold 506 aftercompleting an attempt of bowed rotor mitigation.

The lowest rotor vibration vs. speed in FIG. 8 is for a fullyhomogenized rotor, where mitigation is not necessary (engine parked allnight long, for example). The next higher curve shows a mildly bowedrotor and so on. The maintenance action threshold 506 is a threshold forsetting a maintenance flag such as requiring a troubleshooting routineof one or more system elements. The damage risk threshold 508 may be athreshold to trigger a more urgent maintenance requirement up to andincluding an engine check.

Further, as can be seen in the example of FIG. 8, the motoring band 460is defined for a range of N2 rotor speeds below the critical speed 510and such that the worst case expected vibration 512 is below themaintenance action threshold 506 within the motoring band 460.Performing motoring at a higher speed (e.g., 2000-3000 RPM) but lessthan the critical speed 510 can reduce the time needed for thetemperature homogenization process while also avoiding vibrationsreaching potentially harmful levels.

Accordingly and as mentioned above, it is desirable to detect, preventand/or clear a “bowed rotor” condition in a gas turbine engine that mayoccur after the engine has been shut down. As described herein and inone non-limiting embodiment, the controller 102 may be programmed toautomatically take the necessary measures in order to provide for amodified start sequence without pilot intervention other than theinitial start request. In an exemplary embodiment, the controller 102and/or DSU 104 comprises a microprocessor, microcontroller or otherequivalent processing device capable of executing commands of computerreadable data or program for executing a control algorithm and/oralgorithms that control the start sequence of the gas turbine engine. Inorder to perform the prescribed functions and desired processing, aswell as the computations therefore (e.g., the execution of Fourieranalysis algorithm(s), the control processes prescribed herein, and thelike), the controller 102 and/or DSU 104 may include, but not be limitedto, a processor(s), computer(s), memory, storage, register(s), timing,interrupt(s), communication interfaces, and input/output signalinterfaces, as well as combinations comprising at least one of theforegoing. For example, the controller 102 and/or DSU 104 may includeinput signal filtering to enable accurate sampling and conversion oracquisitions of such signals from communications interfaces. Asdescribed above, exemplary embodiments of the disclosure can beimplemented through computer-implemented processes and apparatuses forpracticing those processes.

While the present disclosure has been described in detail in connectionwith only a limited number of embodiments, it should be readilyunderstood that the present disclosure is not limited to such disclosedembodiments. Rather, the present disclosure can be modified toincorporate any number of variations, alterations, substitutions orequivalent arrangements not heretofore described, but which arecommensurate with the spirit and scope of the present disclosure.Additionally, while various embodiments of the present disclosure havebeen described, it is to be understood that aspects of the presentdisclosure may include only some of the described embodiments.Accordingly, the present disclosure is not to be seen as limited by theforegoing description, but is only limited by the scope of the appendedclaims.

1. A bowed rotor start mitigation system for a gas turbine engine, thebowed rotor start mitigation system comprising: a controller operable toreceive a speed input indicative of a rotor speed of the gas turbineengine and a measured temperature of the gas turbine engine, thecontroller further operable to drive motoring of the gas turbine engineby oscillating the rotor speed within a motoring band for a motoringtime based on the measured temperature when a start sequence of the gasturbine engine is initiated.
 2. The bowed rotor start mitigation systemas in claim 1, wherein the motoring band comprises a range of speedsbelow a resonance speed of the gas turbine engine.
 3. The bowed rotorstart mitigation system as in claim 1, wherein the measured temperatureis adjusted with respect to a measured ambient temperature of the gasturbine engine.
 4. The bowed rotor start mitigation system as in claim1, wherein the motoring time is associated with a bowed rotor riskparameter that is determined based on the measured temperature.
 5. Thebowed rotor start mitigation system as in claim 1, wherein the measuredtemperature is determined based on reading one or more temperaturesensors of the gas turbine engine for a predetermined period of timewhen the start sequence of the gas turbine engine is initiated.
 6. Thebowed rotor start mitigation system as in claim 1, wherein based ondetermining that bowed rotor mitigation is complete, the controller isoperable to monitor the vibration level while sweeping through a rangeof rotor speeds including a critical rotor speed and determine whetherbowed rotor mitigation was successful prior to starting the gas turbineengine.
 7. The bowed rotor start mitigation system as in claim 1,wherein the measured temperature is determined based on reading datafrom one or more temperature sensors at station 3 of the gas turbineengine.
 8. The bowed rotor start mitigation system as in claim 1,wherein the measured temperature is determined based on reading datafrom one or more temperature sensors at station 4 of the gas turbineengine.
 9. The bowed rotor start mitigation system as in claim 1,wherein the measured temperature is determined based on reading datafrom one or more temperature sensors at station 4.5 of the gas turbineengine.
 10. A gas turbine engine system comprising: a motoring systemoperable to drive rotation of the gas turbine engine; a speed pickup; atemperature sensor; and an electronic engine control operable to receivea speed input from the speed pickup indicative of a rotor speed of thegas turbine engine and a measured temperature from the temperaturesensor, the electronic engine control further operable to drive motoringof the gas turbine engine by controlling the motoring system tooscillate the rotor speed within a motoring band for a motoring timebased on the measured temperature when a start sequence of the gasturbine engine is initiated.
 11. The gas turbine engine as in claim 10,wherein the motoring band comprises a range of speeds below a resonancespeed of the gas turbine engine and the measured temperature is adjustedwith respect to a measured ambient temperature of the gas turbineengine.
 12. A method of bowed rotor start mitigation for a gas turbineengine, the method comprising: receiving, by a controller, a speed inputindicative of a rotor speed of the gas turbine engine; receiving, by thecontroller, a measured temperature of the gas turbine engine; anddriving, by the controller, motoring of the gas turbine engine byoscillating the rotor speed within a motoring band for a motoring timebased on the measured temperature when a start sequence of the gasturbine engine is initiated.
 13. The method as in claim 12, wherein themotoring band comprises a range of speeds below a resonance speed of thegas turbine engine.
 14. The method as in claim 12, wherein the measuredtemperature is adjusted with respect to a measured ambient temperatureof the gas turbine engine.
 15. The method as in claim 12, wherein themotoring time is associated with a bowed rotor risk parameter that isdetermined based on the measured temperature.
 16. The method as in claim12, wherein the measured temperature is determined based on reading oneor more temperature sensors of the gas turbine engine for apredetermined period of time when the start sequence of the gas turbineengine is initiated.
 17. The method as in claim 12, wherein based ondetermining that bowed rotor mitigation is complete, the controller isoperable to monitor the vibration level while sweeping through a rangeof rotor speeds including a critical rotor speed and determine whetherbowed rotor mitigation was successful prior to starting the gas turbineengine.
 18. The method as in claim 12, wherein the measured temperatureis determined based on reading data from one or more temperature sensorsat station 3 of the gas turbine engine.
 19. The method as in claim 12,wherein the measured temperature is determined based on reading datafrom one or more temperature sensors at station 4 of the gas turbineengine.
 20. The method as in claim 12, wherein the measured temperatureis determined based on reading data from one or more temperature sensorsat station 4.5 of the gas turbine engine.